Wavelength division multiplexing (WDM) systems typically comprise multiple separately modulated laser diodes at the transmitter. Each diode generates the signal associated with one of the channels in the WDM signal. These laser diodes are tuned to operate at different wavelengths. When combined in an optical fiber, the WDM optical signal comprises a corresponding number of spectrally separated channels within a signal band. At the receiving end, the channels are usually separated from each other using thin film filter systems, to thereby enable detection by separate photodiodes.
WDM technology enables the collective amplification of the various the channels in gain fiber, such as erbium-doped fiber and/or regular fiber, in a Raman amplification scheme.
Other WDM applications include the dynamic routing of channels in optical WDM networks with multiple network access nodes.
In commercially available and proposed WDM systems, the channel assignments/spacings can be tight, 100 GigaHertz (GHz) to 50 GHz, based on the ITU grid. Further, the number of potential channels on a link can be large. Observation of the ITU Grid suggests 100""s of channels in the Lxcex1, Cxcex1 and Sxcex1, bands, even if the 50 GHz offset of the Lxcex2, Cxcex2, and Sxcex2 band is ignored. Still other systems are being proposed that have assignments/spacings in the 10 to 25 GHz range. Thus, each channel must be confined to its channel slot frequency assignment to an absolute accuracy of less than 10 GHz, in some cases.
In order to verify the proper operation of these WDM systems, optical channel monitors are required. These devices typically have a tunable band pass filter that is scanned across the signal band to detect the individual channels. It can thus verify that channels have proper wavelength positions and that proper guard bands are being maintained between adjacent channels. They can also be used to verify that the channel powers are consistent with each other such that one channel is not broadcasting with a power that is overwhelming adjacent channels.
In some applications, it is further desirable to have the ability to resolve the absolute wavelengths of the channels. This typically requires some sort of wavelength reference signal.
Although some systems use capacitive sensing to infer the center wavelength position of the tunable filter passband, other typically more accurate systems rely on optical reference signals.
The filter is scanned across a reference signal with a known and highly stable spectral feature, such as a frequency locked line of a distributed Bragg reflector laser. This is used to calibrate the tunable laser for a subsequent scan across the signal band of the WDM signal. From this information, the channel monitoring system either extrapolates or interpolates the absolute frequency scale in the WDM signal from the spectral feature in the reference signal.
The present invention is directed to a tunable filter system, which is preferably used as an optical channel monitor in a WDM system, although it has applicability in any tunable filter application requiring a wavelength reference.
The invention relies on a characteristic of a class of tunable filters, such as Fabry Perot etalons, in which light that is not transmitted through the filter is reflected. Thus, while the tunable filter appears as a band pass filter in transmission, it functions as a notch filter, in reflection. Preferably, a reflection detector is provided that detects spectral features of a reference signal that are reflected by or otherwise returned from the tunable filter.
In general, according to one aspect, the invention features a tunable filter system that comprises a signal source providing a WDM having multiple channels, or other signal requiring spectral analysis, within a spectral signal band. A reference signal source is also provided that generates a reference signal with spectral features, such as narrow spectral lines, that are located within a spectral reference band. A tunable filter functions as a band pass filter in transmission and a notch filter in reflection. It is controlled to filter both the reference signal and the WDM signal. A transmission detector is provided to detect the signal transmission through the tunable filter and a return detector is provided to detect the signal that is returned from the tunable filter.
In the preferred embodiment, the spectral reference band is located outside and separate from the signal band. The bands are separated from each other, relative to the tunable filter""s free spectral range such that one order of operation of the tunable filter scans the signal band while another order of operation scans the reference band. In the preferred embodiment, the tunable filter is a Fabry-Perot tunable filter or similar device that has an optical resonant cavity bounded by at least two reflectors. At least one of these reflectors is a deflectable membrane to thereby provide for the tunable spectral passband/notch.
In the preferred embodiment, the controller monitors the transmission detector to detect the multiple channels within the signal band and monitors the reflection detector to detect the reference features in the reference band. The position of these reference features is preferably known on an absolute basis. The controller uses the information from the reflection detector to determine the passband""s absolute wavelength within the signal band. Typically, fixed bandpass or low/high pass filters are provided between the tunable filter and the transmission detector and between the tunable detector and the reflection detector. This ensures that the transmission detector, for example, only sees the signal band and the reflection detector, for example, only sees the reference band.
In the present implementation, the reference source is comprised of a broadband source such as a luminescent light emitting diode. A fixed wavelength etalon is then provided in combination with a band pass filter that defines the reference band.
In general, according to another aspect, the invention also features a WDM signal analysis method. This comprises receiving a WDM signal having multiple channels within a spectral signal band while generating a reference signal having spectral reference features located in a spectral reference band. The WDM signal and the reference signal are launched at a tunable filter. A passband of the tunable filter is scanned across the signal band. Multiple channels within the signal band are detected by reference to transmission through the tunable filter, while an absolute wavelength of the multiple channels is determined by reference to spectral features in the reference band that is detected in reflection from the tunable filter.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.